1. Field of the Invention
The present invention relates to a laminate and a thin film solar cell including the same. More specifically, the present invention relates to a polyimide laminate that is processable at high temperatures of at least 550° C. and can be used to fabricate a thin film solar cell with high flexibility and improved energy conversion efficiency due to its excellent durability and barrier properties, and a thin film solar cell including the laminate.
2. Description of the Related Art
A thin film solar cell using a copper.indium.selenium (CIS)-based or coppe.indium.gallium.selenium (CIGS)-based compound semiconductor as a material for a light-absorbing layer has high photoelectric conversion efficiency. In comparison with other types of solar cells, the thin film solar cell is advantageous in that the light-absorbing layer can be formed into a thin film having a thickness on the order of hundreds of nanometers to a few micrometers, which greatly reduces the amount of the material used. The thin film solar cell has received attention due to the advantage of low fabrication cost.
General thin film solar cells have a structure in which a metal electrode, a light-absorbing layer, and a transparent electrode are sequentially laminated on a substrate. Most conventional thin film solar cells use soda-lime glass substrates. With the recent increasing need for flexible thin film solar cells, flexible films have been used as substrates. In comparison with conventional thin film solar cells using glass substrates, thin film solar cells using flexible films as substrates have a wide range of applications and can be fabricated by roll-to-roll processing, which is suitable for large-scale fabrication, because of their high flexibility and light weight.
In recent years, considerable research efforts have concentrated on the development of polyimides as materials for the formation of flexible films. Polyimides have excellent mechanical properties, heat resistance, chemical resistance, and electrical insulation performance. Due to these advantages, polyimides can be widely used in various films for electronic devices and optical waveguides, such as interlayer insulating films for semiconductors, buffer coats, flexible printed circuit boards, and liquid crystal alignment films.
However, thin film solar cells using polyimide substrates have lower energy conversion efficiency than thin film solar cells using glass substrates and require baking at high temperatures of at least 450° C. to prevent the formation of defects in light-absorbing layers. However, it is difficult to heat the polyimide substrates to 450° C. or above because the bake-out temperature of polyimides is about 450° C. When a polyimide substrate is baked at a high temperature of 500° C. or above to fabricate a thin film solar cell, it suffers from the problem of warpage or poor mechanical properties and the electrodes or the light-absorbing layer is apt to crack.
Numerous methods have been proposed to solve the problems encountered in the fabrication of thin film solar cells using polyimide films as substrate materials. These methods are associated with the attachment of a barrier film to the back side of a polyimide film and the use of a metal as a substrate material. Another method is to use, as a substrate, a polyimide film whose coefficient of linear expansion and tensile strength at break are improved so as to meet the requirements of a flexible substrate, such as high transparency, low thermal expansion, and high glass transition temperature.
However, polyimide films with very high heat resistance sufficient to withstand high temperature baking and good dimensional stability have not yet been developed. The method associated with the formation of a barrier layer at the back side of a polyimide film has the problem that the polyimide film tends to be bent or cut upon high temperature baking. Although the method associated with the use of a metal base layer as a substrate for a thin film solar cell is advantageous in that the metal base layer is processable at high temperatures of 500° C. or above in the fabrication of the solar cell, thus suppressing the formation of defects in the light-absorbing layer, it has the problems that the energy conversion efficiency of the solar cell are deteriorated, it is difficult to carry out a monolithic process, and a process for forming a barrier film is inevitably required due to high surface roughness of the metal base layer and the presence of impurities in the metal.